Lithofacies analysis of the Lower Cretaceous deposits of the Vankorskoye oil-and-gas field (Western Siberia)

The paper tells about the results of the lithofacies analysis of the coastal-marine deposits of the Yakovlevskaya and Nizhnehetskaya Suites (the Lower Cretaceous) of the oil-and-gas field in Vankorskoe. This analysis was based on the lithologic and petrographic studies of core samples and grain size analysis in slides. The analysis results allowed drawing the correlation pattern of the Yakovlevskaya and Nizhnehetskaya Suites of boreholes, which were studied. According to the granulometric data, the cumulative curves, the curves of differential grain size distribution and the G.F. Rozhkov's dynamogenetic ά-τ diagram were drawn. These data allowed getting straight the conditions of sedimentation of suites. The Yakovlevskaya suite was forming in the continental conditions. In the point of Vankorskaya-10 sedimentation was in the near-bottomland plots, floodplain lakes and bogs (the form of the SP curve, the low and the high hydro-dynamic regime, the abundance of carbonic recrements). Apparently, this part of the area of interest was bogged up for a long time. In the point of borehole-9 sedimentation seems to be in rivers. The abrupt change in the hydrodynamic regime, the coarse wastes, the poorly continuous (by the author's data) grain composition and also the vegetation remains prove such conditions of formation. A paleochannel of a large river with a high hydrodynamic level might have existed sometime at the studied area. During the Berriasian-Early Valanginian Time, the formation of deposits on the studied territory is supposed to take place in the coastal-marine conditions in various facies conditions - from a shallow-water maritime gulf (mainly with argillaceous-silt stones) to a littoral area (aleuropelitic sediments with lenticular sandstone strata beds). At that time aleuro-argillous littoral sediments with strata and lenses of arenaceous beds of offshore beaches, swells and swallows (the Nizhnehetskaya Suite) were mainly formed in the area of the modern near-edge zone swells and monoclines. Consequently, the productive sedimentary deposits of the Nizhnehetskaya Suite were formed within the Vankorskoe raising in the coastal-maritime conditions and in the facies conditions of alongshore sandbars and barrier islands. The productive deposits of the Yakovlevskaya Suite were formed in the continental settings and facies conditions of the side bed, the mainstream, the beach barriers. Comparing the results of the granulometric analysis and the data of the previous exploration on sedimentation conditions of the Lower Cretaceous deposits in the Vankorskoe field,it must be admitted that the most effective and more correct facies diagnostics may be achieved by combining granulometric analysisand detailed lithofacies exploration.

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